Magneto-optical recording media have been becoming, put to practical use as high-density, low-cost rewritable information recording media.
Particularly, those magneto-optical recording media having a recording layer comprising an amorphous alloy of a rare earth metal and a transition metal exhibit very excellent characteristics.
A major drawback of magneto-optical recording media left to overcome is that the media are not capable of overwriting (overwrite recording). That is, the conventional magneto-optical recording media require an erasing process before recording, and, accordingly, one recording operation requires two rotating operations, thereby lowering the rate of data transfer.
In recent years, a number of overwriting methods for magneto-optical recording media have been proposed.
A promising one of the methods proposed is an optical modulation overwriting method in which a multilayer film is utilized. This system, as is discussed in the Abstracts of Papers for the 34th Oyo-Butsurigaku Kankei Rengo Koenkai, 28P.ZL-3, P721 (1987), comprises a perpendicular magnetization layer (recording layer) having a low Curie temperature and a high coercive force and a perpendicular magnetization layer (auxiliary layer) having a relatively higher Curie temperature and a lower coercive force. Overwriting is carried out by first applying a magnetic field (initializing magnetic field) having sufficient intensity for aligning the magnetization directions in the auxiliary layer but having no influence on the recording layer, and then irradiating with a beam of light modulated to have two values of power, a high power (P.sub.H) and a low power (P.sub.L), while applying a bias magnetic field.
Upon P.sub.L irradiation, inversion of the magnetization direction does not occur in the auxiliary layer, and the magnetization directions in the recording layer are oriented in a stabilizing direction through switched connection with the auxiliary layer. Upon P.sub.H irradiation, the auxiliary layer undergoes inversion of magnetization directions by a bias magnetic field, and as a result thereof, the recording layer is accordingly oriented oppositely to the case of P.sub.L irradiation, whereby overwriting can be achieved.
One of the problems involved in this system is that the recording medium must be designed by considering a sufficient power difference between the P.sub.L and the P.sub.H. If the difference is insufficient, high-power recording may take place at a high-temperature portion in the center of a beam spot when P.sub.L recording is carried out. This arises from the temperature distribution in the beam spot, and becomes more conspicuous when a more intense bias magnetic field is applied.
In addition, diffusion of heat from a P.sub.H region causes a heat gradient in a P.sub.L region, resulting in a reduced power margin for P.sub.L. By the "power margin" herein is meant a power range in which sufficient C/N ratio and erasing ratio can be obtained over the entire frequency range of the recording signals.
The requirement for a sufficient difference between P.sub.L and P.sub.H means that the P.sub.H should not be lowered, i.e., a high-power laser is required.
For high-speed rotation of a disk, on the other hand, the P.sub.H should be lowered in view of laser power limitations, and it is desirable to use an auxiliary layer having a low H.sub.C2, which has been impossible due to the above-mentioned problems.